Isotopic densification of propellant

ABSTRACT

A propellant and a propellant combination. In a preferred embodiment, the propellant includes a portion of an element and two isotopes of the element. The portion of the element is enriched with one of the isotopes and is adapted to being ionized. Preferably, the element is xenon. In another preferred embodiment, the present invention also provides a propellant combination that includes portions of two elements that are adapted to being combined. The portion of the first element includes two isotopes and is enriched in the first of the isotopes. Preferably the first element is fluorine, chlorine, boron, or bromine. Methods of using isotopically enriched propellants are provided in other preferred embodiments as well as mobile platforms (e.g. spacecraft) that are adapted to use isotopically enriched propellants.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation in part of co-owned, co-pending U.S. patent application Ser. No. 10/988,055, filed by Tillotson on Nov. 12, 2004, entitled Isotopic Lightening, and incorporated in this application as if set forth in full.

FIELD OF THE INVENTION

This invention relates generally to chemicals used as propellants and, more particularly, propellants having relatively high specific impulse.

BACKGROUND OF THE INVENTION

One enduring problem in spacecraft design is to minimize the non-payload weight of the spacecraft, i.e. the mass of the spacecraft structure and the expendables (e.g. propellant) stored on the spacecraft. This problem arises because every extra kilogram (kg) of mass in the spacecraft means less mass allocation available for the payload. Given that the cost of delivering payloads to space is high (from about $10K/kg for bulk deliveries to low Earth orbit to upwards of $1 million/kg for hardware soft-landed on Mars) there is a powerful incentive to reduce the mass of the payloads, the launch vehicle, the spacecraft, the constituent components of these devices, and the onboard expendables. Another enduring problem is the need to minimize the total volume of the spacecraft. Similar to the situation with the mass of the spacecraft, every cubic centimeter devoted to the spacecraft reduces the volume available to the payload. Thus, a need also exists to minimize the volume devoted to the propellants and spacecraft structure.

A common way to address the mass and volume challenges is through the development of new propellants. New propellants, though, must meet or exceed the performance requirements associated with the propellants that are being replaced. Also, the new propellants must be chemically compatible with existing fluid systems and rocket engines. If the new propellants are not compatible, then the systems and engines may need redevelopment in order to use the new propellant. Additionally, because of the harsh environments created within typical engines, each new propellant must be characterized for a variety of properties and conditions such as thermal stability, shock stability, ignition pressure surges, combustion instability, catalyst bed poisoning, and the like. Further, as chemicals, these propellants (and related ground systems) must meet stringent environmental regulations. For these reasons, the development of a new propellant is an expensive undertaking.

Thus, a need exists for propellants that are similar to existing propellants in every way, except that they have a different density.

SUMMARY OF THE INVENTION

The present invention provides, inter alia, methods of using isotopically enriched materials for propellants. The materials may be enriched in heavier isotopes and depleted in lighter isotopes. Enriching a propellant with heavier isotopes yields a propellant that is denser than the unenriched propellant. Otherwise the enriched propellant has properties that are identical to the properties of the unenriched propellant. These embodiments are well suited for use in electric propulsion systems.

In the alternative, the material may be enriched in lighter isotopes and depleted in heavier isotopes. Enriching a propellant with lighter isotopes yields a propellant that is less dense than the unenriched propellant. Otherwise the enriched propellant has properties that are identical to the properties of the unenriched propellant. These embodiments are well suited for use with thermal (chemical) propulsion systems. Additionally, the present invention provides mobile platforms (e.g. satellites, spacecraft, or launch vehicles) that are adapted to use isotopically enriched propellants.

In a first preferred embodiment, the present invention provides a propellant. The propellant includes a portion of an element with two isotopes each having a natural abundance in the element. The portion of the element is enriched with the first isotope and is adapted to be ionized. Preferably, the element is xenon.

In a second preferred embodiment, the present invention provides a method of operating a mobile platform. The method includes enriching a portion of an element with a first isotope of the element, ionizing the enriched portion of the element, and using the enriched and ionized portion of the element to produce thrust. Preferably, the method includes sizing a propellant tank based on the enriched portion of the element. Also, the enriching of the propellant may densify the propellant or may increase the specific impulse of the ionized propellant. Further, the enriched propellant may be stored on the mobile platform. The method preferably includes performing a cost/benefit analysis to determine an enrichment level for the propellant.

In a third preferred embodiment, the present invention provides a mobile platform including a rocket engine and a propellant container that is sized and dimensioned to store a propellant. The propellant includes a portion of an element with two isotopes each having a natural abundance in the element. The portion of the element is enriched with the first of the isotopes and is adapted to be ionized. The rocket engine communicates with the propellant container to receive and ionize the enriched propellant. Preferably, the mobile platform is a geosynchronous satellite with a xenon electric propulsion system.

In yet another preferred embodiment, the present invention provides a mobile platform. The mobile platform includes a propellant container and a rocket engine. The propellant container is sized and dimensioned to store a first mass of a propellant that has natural isotopic abundances. The container, though, stores a second mass of a propellant that is enriched in an isotope of the propellant. Preferably, the density of the enriched propellant is greater than the density of the natural propellant. Additionally, the rocket engine can ionize the enriched propellant.

In still another preferred embodiment, the present invention provides a propellant combination. The propellant combination includes portions of a first and a second element. The portion of the first element is isotopically enriched and is adapted to combine with the portion of the second element. Preferably, the first element is chlorine, boron, or bromine.

In another preferred embodiment, the present invention provides still another mobile platform. The mobile platform of the present invention includes two propellant containers and a rocket engine. The first of the containers is sized and dimensioned to store a first isotopically enriched propellant while the second container is sized and dimensioned to store a second propellant. The rocket engine communicates with both propellant containers and is adapted to chemically combine the propellants.

Yet another mobile platform is provided by the present invention in another preferred embodiment. The mobile platform of the current embodiment includes two propellant containers and a rocket engine. Both propellant containers are sized and dimensioned to store their respective propellants that have natural isotopic abundances. The rocket engine communicates with the propellant containers to receive and combine the propellants. However, one of the propellants stored in the containers is isotopically enriched.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 illustrates a mobile platform constructed in accordance with a preferred embodiment of the present invention;

FIG. 2 schematically illustrates a propellant system of the mobile platform of FIG. 1;

FIG. 3 illustrates another mobile platform constructed in accordance with a preferred embodiment of the present invention; and

FIG. 4 illustrates a method in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings in which like reference numbers indicate like elements, FIG. 1 illustrates a mobile platform (e.g. a satellite) constructed in accordance with a preferred embodiment of the present invention.

The exemplary satellite 10 illustrated by FIG. 1 is a Boeing 702 satellite (available from The Boeing Company of Chicago, Ill.). The satellite 10 includes two major subassemblies: a payload 12 and a bus 14. The payload 12 typically includes telecommunications, remote sensing, or scientific research equipment although the payload 12 could be any type of payload. It is the capabilities provided by the payload 12 that usually justify the cost of assembling and launching the satellite 10. As a result, the provider of the satellite has a strong incentive to maximize the allocation of mass and onboard volume for the payload.

The bus 14, however, cannot be neglected because it contains many systems that support the payload 12. These support systems include a solar energy subsystem 16, a structural subsystem 18, and a propulsion system 20. During ascent from Earth on a launch vehicle (such as the Delta IV, Evolved, Expendable, Launch Vehicle that is also available from The Boeing Company), the structural subsystem 18 carries the loads imposed on the satellite 10. On orbit, the power subsystem supplies 16 the payload 12 and bus 14 with regulated power so that the satellite 10 can accomplish its mission.

The propulsion system 20 supports the payload 12 by allowing the satellite 10 to maneuver for such purposes as stationkeeping, orbital adjustments, and avoidance of space debris. To accomplish these maneuvers, the propulsion system 20 includes propellant tanks 22 and rocket engines 24 connected to the tanks 22 by propellant lines (not shown). Recently, The Boeing Company has outfitted many satellites 10 with Xenon Ion Propulsion Systems (XIPS) 20. A typical XIPS system 20 is illustrated schematically in FIG. 2 and includes several propellant tanks 22, several engines 24, a power supply 26, and propellant lines 28. The power supply 26 is a high voltage D.C. power supply and is connected to the engine 24. When it is desired to provide thrust to the satellite 10, the power supply 26 is turned on so that an ionization chamber 28 of the engine 24 is electrically excited. Xenon propellant then flows to the engine 24 where it is ionized in the chamber 28, accelerated by an electrically charged grid, and directed out of the engine 24 by the grid. Because the engine 24 accelerates the xenon ions as they are expelled from the satellite 10, a reaction (i.e. a force) develops on the satellite 10 thereby creating thrust on the satellite 10. These XIPS systems 20 are an order of magnitude more efficient at creating thrust than typical liquid fuel propulsion systems. Nonetheless, the amount of propellant carried onboard the bus 14 often determines the useful life of the satellite 10.

With reference again to FIG. 1, for mass allocation purposes the satellite 10 is typically divided between the propellant and the “dry” structure (i.e. the structure and subsystems of the satellite). The dry weight is further divided between the payload 12 and the bus 14 portions of the satellite 10. Since the payload 12 accomplishes the purposes for which the satellite 10 was launched, it is desirable to maximize the fraction of the mass that is allocated to the payload 12. The propellant, however, typically consumes a large fraction of the overall mass allowed for the entire satellite 10, thereby leaving little mass available to allocate between the payload 12 and the bus 14. The mass allocation for the payload 12 could therefore be increased by decreasing the mass of the propellant but at the expense of shortening the maneuvering life of the satellite 10.

In accordance with the principles of the present invention, an alternative to trading mission life for payload 12 mass is using isotopic enrichment to increase the density of the propellant(s). Since the propellant is made denser by enrichment, the propellant tanks 22 and associated structure can be smaller and less massive than conventional propellant tanks and associated structure. The increase in density operates to decrease the size of the propellant tanks as follows. The approximate delta velocity (ΔV) imparted to a vehicle by a mass of propellant (M_(prop)) is given by ΔV=V _(ex)*ln(m _(initial) /m _(final)) where V_(ex) is the exhaust gas velocity, m_(initial) is the initial mass of the vehicle, and m_(final) is the final mass of the vehicle. Of course m _(initial) =m _(final) +m _(propellant). Therefore, for a given delta velocity and a given exhaust velocity, a certain mass of propellant will be required. If the propellant is an isotopically densified gaseous propellant, then fewer atoms (or molecules) of the propellant are required to yield the required propellant mass. As a result, fewer moles of the gaseous propellant are required. Since the volume of a quantity of gas depends on the number of moles of the gas, the size of the propellant tank (for a given gas, pressure, and temperature) is reduced accordingly for the densified propellant as compared with the same propellant that has not been isotopically densified, Moreover, the resulting increase in the average mass of the propellant atoms thereby lowers the energy required to ionize a given mass of propellant. Because fewer ions are needed to yield the same propellant mass, the invention improves the efficiency of ion and plasma-dynamic thrusters 24 typically used in electric propulsion systems 20. The increased efficiency, in turn, reduces the power system's mass which allows the satellite to have yet smaller tanks or a longer maneuvering lifetime. Moreover, as the propellant tank decrease in size, the moment of inertia of the vehicle decreases. As a result, the size of the attitude control system and the amount of propellant used for attitude control also decreases.

Those skilled in the art will also understand that the denser propellants provided by the present invention are more efficient at converting the electric energy (provided by an ion engine) to exhaust gas kinetic energy. This result occurs because the ionization energy per atom of propellant remains the same while the kinetic energy per atom for a given exhaust velocity increases with increasing atomic weight. In some applications it may be desirable to increase the voltage of the accelerating grid of the ion engine to impart the increased kinetic energy to the exhaust ions. The increase in the grid voltage can be determined as follows. First exhaust gas velocity, V_(ex), is given by: Vex=√{square root over ((2*E/m))} and E=qV Where E is the kinetic energy, m is the ion mass, q is the amount of charge on the exhaust gas, and V is the grid potential. Since higher mass requires proportionally higher kinetic energy, the grid potential (V) increases in proportion to the ion mass. Even though there may be an increase in the weight of the electrical insulation (to account for the somewhat higher voltage), the higher voltage also means that the necessary current will decrease. So any increase in the mass of the insulation will be more than offset by the smaller radiator required to reject waste heat (generated by IR² resistance heating) from the system. On the other hand, the propellant for an ion engine can be enriched with a lighter isotope which would give a higher I_(sp) at the same voltage, but would make the propellant less dense thereby offsetting some of the weight savings associated with the higher I_(sp).

Thus, according to a preferred embodiment of the present invention, at least one element that is a constituent of a propellant is enriched with at least one heavier isotope. More particularly, those elements that

1.) have a large relative variance in their isotopic mass and

2.) have enough of both light and heavy isotopes that changing the concentration of the isotopes in favor of the heavier isotopes causes a change in density of the element are isotopically enriched for use as a propellant. Of course, the present invention is not limited to just those cases in which the enrichment can be made economically because, in some applications, the benefits of a density increase may be of greater moment than mere economic considerations. For instance, isotopic densification may allow some missions that would otherwise not be possible.

In particular, xenon, lithium, and carbon are well suited for isotopic enrichment to improve the propulsion system 20 performance. More particularly, xenon, the propellant used in The Boeing Company's 702 satellites 10 (see FIG. 2), has several isotopes that include ¹²⁹Xe (26.4%), ¹³¹Xe (21.2%), ¹³²Xe (26.9%), ¹³⁴Xe (10.4%), and ¹³⁶Xe. Thus, as discussed in co-owned, co-pending U.S. patent application Ser. No. 10/988,055, entitled Isotopic Lightening (which is incorporated in this patent application as if set forth in full) enriching xenon with its heavier isotopes by a factor of about ten is achievable and increases the overall density of the xenon by about 2.4%. One exemplary mission on which isotopically enriched xenon could be used is NASA's Jupiter Icy Moons mission. Because the propellant tanks will need to store ten tons of xenon, a 2.4% increase in the density of the xenon will save a significant amount of tank (and related structure) mass and volume. The reductions in dry mass and tank volume would allow NASA to place that much more scientific payload in the vicinity of Jupiter and its icy moons.

Also, lithium has two isotopes ⁶Li and ⁷Li with natural abundances of, respectively, 7.59% and 92.41%. Thus, a density increase of about 1% is achievable. Further, lithium has been tested as a propellant for electric thrusters (e.g. engines 24) thereby providing a high level of assurance that enriched lithium should perform well as a propellant.

Similarly, carbon has been proposed as a candidate propellant for future ion propulsion systems 20. The particular form of carbon proposed for this use is known as Buckminsterfullerene (e.g. C₆₀ or C₇₀). Buckminsterfullerene possesses a large molecular mass and low ionization energy which makes it a good candidate for isotopic enrichment. Carbon also has two isotopes ¹²C (with a natural isotopic abundance of 98.89%) and ¹³C (natural abundance of 1.11%). Further, the natural abundance of the heavier ¹³C isotope is large enough that enrichment of carbon by a factor of ten is practicable and yields almost a 1% increase in density.

Turning now to FIG. 3, another mobile platform constructed in accordance with the principles of the present invention is illustrated. The mobile platform (the Space Shuttle) 100 includes several large sub-assemblies including the Orbiter 102, the External Tank (ET) 104, and a pair of Solid Rocket Boosters (SRBs) 106 and 108. The Orbiter 102 and the SRBs 106 and 108 attach to the External Tank 104 as shown in FIG. 3. The External Tank 104 feeds liquid propellants to the three Space Shuttle Main Engines (SSMEs, not shown) on the Orbiter 102. Together with the SRBs 106 and 108, the SSMEs provide thrust to propel the Orbiter 102 into Earth orbit. The SSMEs and the SRBs operate by combining chemical propellants in highly exothermic reactions to produce hot gases. These gases are then accelerated through nozzles of the SSME and the SRB rocket engines to produce the thrust desired to reach orbit. As is well known, the efficiency of the rocket engines (and the propellants) is measured by the specific impulse (I_(sp)) of the engines. In such chemical propulsion systems, propellant combinations (e.g. an oxidizer and a fuel) that generate exhaust gases having lower molecular weights possess higher I_(sp) than combinations that produce exhaust gases having higher molecular weights.

To provide for these lighter exhaust gases, the current embodiment includes at least one propellant that is isotopically enriched in a lighter isotope. When the raw propellants combine in a rocket engine, the resulting atoms and molecules that include the lighter isotopes become constituents of the exhaust gas. As a result, the exhaust gas has a lower average molecular weight than exhaust gases produced from conventional propellants. Thus, the I_(sp) of the propellant combinations (and engines adapted to use the isotopically enriched propellants) are higher than the specific impulses of previously available propellant combinations. Moreover, because of the improved I_(sp,) a smaller mass of propellant is needed to achieve the same delta-V as achieved with unenriched propellant.

Exemplary liquid propellants that contain elements suitable for isotopic lightening include chlorine trifluoride (ClF₃), triethylboron (C₆H₁₅B), bromine pentafluoride (BrF₅), and pantaborane (B₅H₉). The exemplary elements include boron, bromine, and chlorine which have the following isotopes (and natural abundances): Boron ¹⁰B (19.80%) ¹¹B (80.20%) Bromine ⁷⁹Br(50.69%) ⁸¹Br (49.31%) Chlorine ³⁵Cl (75.77%) ³⁷Cl (24.23%)

In addition to enriched liquid propellants, the present invention also provides enriched solid propellants. In particular, a common solid propellant ammonium perchlorate used in the SRBs 106 and 108 of the Space Shuttle 100 (FIG. 3) can be isotopically lightened because chlorine contributes to the weight of the propellant. The SRB propellant includes about 70% ammonium perchlorate, about 16% of aluminum, about 12.% acrylic acid acrylonitrile, about 2% epoxy curing agent, and about 0.07% of iron oxide catalyst. These chemicals are mixed together and poured into the casing of the SRB 106 and 108 where they cure to form the solid propellant. Given that the molecular weight of ammonium perchlorate is about 69.4, (about half of which is provided by the chlorine atom), and that each SRB 106 and 108 contains about 1,100,000 pounds of propellant, the weight savings from isotopically enriching the chlorine to 87.5% ³⁵Cl amounts to a weight savings of about 8,000 pounds (4 tons) per shuttle flight assuming that the slightly lower total impulse available from the isotopically lightened propellant is acceptable for the particular mission. To place this in perspective, consider that the Shuttle payload capacity is about 65,000 pounds.

If it is desired to provide the same total impulse as provided by the non lightened propellant then additional moles (and volume) of propellant will be required. The reason that lightening the propellant reduces the total impulse is that the impulse depends on the mass of the exhaust gas as shown by the following equation: v _(ex)=sqrt(2*E _(c) /m _(avg)) where v_(ex) is the exhaust gas velocity, E_(c) is the combustion energy per molecule, and m_(avg) is the average mass of molecules in the exhaust. Further, the total impulse depends on the average mass as follows: P=N*m _(avg) *v _(ex) Or, rewriting the equation: P=N*sqrt(2*E _(c) *m _(avg))

where N is the number of molecules in the exhaust. Thus, if m_(avg) is 0.66% lower (which it will be if the chlorine is enriched to 87.5% ³⁵Cl), N will need to increase by about 0.33% to get the same total impulse, P. The net result is that, even with the desire to maintain the total impulse available, a mass of enriched propellant approximately equal to 99.67% of the mass of the un-enriched propellant is required. Thus, the weight savings per Shuttle flight will still be about 2635 pounds. An additional savings will also be available because propellant is not required to lift the 2635 pounds saved. Thus, the savings will be somewhat more than 2635 pounds.

Turning now to FIG. 4, a flowchart of a method in accordance with a preferred embodiment is illustrated. Generally, the method includes manufacturing an enriched propellant, loading the enriched propellant on to a mobile platform, and using the enriched propellant to propel the platform. As shown in FIG. 4, the method includes determining an enrichment level for at least one of the elements of a propellant. See operation 202. The propellant may be a gas, liquid, or solid. Further, the propellant may be intended for use in an electric or chemical engine. Regardless of the physical state of the propellant or its intended use, the enriched propellant allows for improvements in the overall performance of the platform, decreases in the overall weight of the vehicle (particularly at liftoff), and lower overall expenses associated with operating the platform. To optimize the benefits provided by the enriched propellant, a trade study, cost/benefit analysis, or other analysis can performed to determine the enrichment level for a particular system, mission, or mobile platform. The element(s) chosen for enrichment can then be enriched in operation 204. It is useful to note that some elements used as propellants such as xenon and chlorine are gases at room temperature. Other suitable elements such as bromine and carbon can be readily converted to gases near room temperature that can be fed into gaseous diffusion enrichment systems. For instance, bromine can be heated to its boiling point of 138 degrees Fahrenheit with little energy input and carbon can be readily oxidized to carbon monoxide, or carbon dioxide. Carbon is also readily available in the form of methane which is a gas at standard conditions. Methane is therefore well suited for gaseous enrichment. Moreover, because methane (molecular weight 16) is lighter than carbon dioxide (molecular weight 44), the enrichment process will require fewer enrichment cycles and will be correspondingly more efficient than when using carbon dioxide as a feed gas. Therefore, propellants such as carbon and bromine are well suited for enrichment by available gaseous enrichment processes.

Preferably, once the elements are enriched, the compounds constituting the propellant(s) may be manufactured as in operation 206. Since the enriched elements, and compounds, behave similarly to those that are not enriched, the manufacture of the propellants can be via conventional systems and processes. In operation 208 the enriched propellant(s) can be loaded on to a mobile platform (or ground based) propellant tank for storage. When it is desired to use the propellant to produce thrust, appropriate steps are taken to either ionize, or chemically combine, the propellant(s) as in, respectively, operations 210 and 212. Once ionized, or chemically combined, the resulting exhaust gas can then be expelled from an engine to produce the desire thrust. See operation 214.

Various modifications could be made in the constructions and methods described without departing from the scope of the invention. All matter contained in the description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any exemplary embodiment, but should be defined only in accordance with the following claims and their equivalents. 

1. A propellant comprising: a portion of an element; a first isotope of the element; and a second isotope of the element, the first and the second isotopes each having a natural abundance in the element, the portion of the element being enriched with the first isotope and being adapted to be ionized for use as the propellant.
 2. The propellant according to claim 1, further comprising the element including at least one of carbon, lithium, and xenon.
 3. A method of operating a mobile platform, comprising: enriching a portion of an element with a first isotope of the element, the element having a second isotope, the first and the second isotope each having a natural abundance in the element; ionizing the enriched portion of the element; and using the portion of the element as a constituent of a propellant.
 4. The method according to claim 3, further comprising sizing a propellant tank based on the enriched portion of the element.
 5. The method according to claim 3, further comprising the enriching increasing a specific impulse of the enriched and ionized portion of the element.
 6. The method according to claim 3, further comprising the enriching densifying the propellant.
 7. The method according to claim 3, further comprising storing the enriched portion of the element on the mobile platform.
 8. The method according to claim 3, further comprising expelling the enriched and ionized portion of the element from the mobile platform to produce thrust.
 9. The method according to claim 3, further comprising determining an enrichment level for the propellant, the determining including a cost/benefit trade-off.
 10. A mobile platform comprising: a propellant container sized and dimensioned to store a propellant, the propellant to include: a portion of an element, a first isotope of the element, and a second isotope of the element, the first and the second isotopes each having a natural abundance in the element, the portion of the element being enriched with the first isotope; and an engine communicating with the propellant container to receive the enriched propellant from the container and to ionize the propellant.
 11. The mobile platform according to claim 10, wherein the engine is a xenon electric propulsion engine.
 12. The mobile platform according to claim 10, further comprising being a geostationary satellite.
 13. A mobile platform comprising: a propellant container to store a propellant, the propellant to include: a portion of an element, a first isotope of the element, and a second isotope of the element, the first and the second isotopes each having a natural abundance in the element, the propellant container sized and dimensioned to store a first volume of the propellant with the isotopes in their natural abundances; a rocket engine communicating with the propellant container to receive the propellant from the container and to ionize the propellant; and a second volume of the propellant enriched in the first isotope and stored in the propellant container.
 14. The mobile platform according to claim 13, further comprising, the first volume being equal to the second volume and second volume of enriched propellant having more mass than the first volume of the propellant.
 15. A propellant combination, comprising: a first portion of a first element; a first isotope of the first element; a second isotope of the first element, the first and the second isotopes each having a natural abundance in the first element, the first portion of the first element being enriched with the first isotope; and a second portion of a second element, the first and the second portions being adapted to combine.
 16. The propellant combination according to claim 15, further comprising the first element being at least one of chlorine, boron, and bromine.
 17. A mobile platform comprising: at least one propellant container sized and dimensioned to store a first propellant, the first propellant to include: a portion of a first element, a first isotope of the first element, and a second isotope of the first element, the first and the second isotopes each having a natural abundance in the first element, the portion of the first element to be enriched with the first isotope; the at least one propellant container being sized and dimensioned to store a second propellant, the propellants adapted to be combined; and an engine communicating with the at least one propellant container to receive the combined propellants.
 18. A mobile platform comprising: at least one propellant container sized and dimensioned to store a first propellant, the first propellant to include: a portion of a first element, a first isotope of the first element, and a second isotope of the first element, the first and the second isotopes each having a natural abundance in the first element, the at least one propellant container being sized and dimensioned to store the first propellant with the isotopes in their natural abundances; the at least one propellant container being sized and dimensioned to store a second propellant, the first and second propellants to be adapted to be combined; an engine communicating with the at least one propellant container to receive the combined propellants; and a second volume of the first propellant enriched in the second isotope and stored in the at least one propellant container.
 19. The mobile platform according to claim 18, wherein the first element is chlorine.
 20. The mobile platform according to claim 18, wherein the at least one propellant container is a casing.
 21. The mobile platform according to claim 18, wherein the at least one propellant container includes a first propellant tank for storing the first propellant and a second propellant tank for storing the second propellant.
 22. A method of operating a mobile platform, comprising: enriching a first portion of a first element with a first isotope of the element, the first element having a second isotope, the first and the second isotope each having a natural abundance in the first element; combining the enriched first portion of the first element with a second portion of a second element; and using the combined portions to propel the mobile platform.
 23. The method according to claim 22, further comprising the first element being at least one of chlorine, boron, and bromine. 